Shear thinning in deeply supercooled melts

Abstract

We compute, on a molecular basis, the viscosity of a deeply supercooled liquid at high shear rates. The viscosity is shown to decrease at growing shear rates, owing to an increase in the structural relaxation rate as caused by the shear. The onset of this nonNewtonian behavior is predicted to occur universally at a shear rate significantly lower than the typical structural relaxation rate, by approximately two orders of magnitude. This results from a large size-up to several hundred atoms-of the cooperative rearrangements responsible for mass transport in supercooled liquids and the smallness of individual molecular displacements during the cooperative rearrangements. We predict that the liquid will break down at shear rates such that the viscosity drops by approximately a factor of 30 below its Newtonian value. These phenomena are predicted to be independent of the liquid's fragility. In contrast, the degree of nonexponentiality and violation of the Stokes-Einstein law, which are more prominent in fragile substances, will be suppressed by shear. The present results are in agreement with existing measurements of shear thinning in silicate melts.liquids | non-Newtonian | glass transition | rheology N on-Newtonian rheological response of complex fluids undergirds many industries and laboratory procedures, yet because of the extraordinarily broad dynamic range exhibited by these systems, securing microscopic insight-either by direct modeling (1-5) or phenomenological treatments (6-10)-has been difficult. This extraordinary breadth comes about for two somewhat distinct reasons: (i) the broad distribution of lengths of moving units-as in polymers or mixtures of granular and fluid matter-which we may call "built-in heterogeneity"; (ii) the kinetic arrests/slowdowns emerging in otherwise homogeneous systems, owing to cooperative effects at high densities. To begin constructing a systematic microscopic description of non-Newtonian response of complex matter, here, we consider deeply supercooled liquids, which exhibit only the emergent slowdown arising from cooperative, activated transport. By properly accounting for the self-generated cooperativity, we will rationalize the otherwise surprising fact that the onset of shear thinning in supercooled liquids occurs at shear rates that are at least two orders of magnitude lower than the typical structural relaxation rate (11). In addition, we will compute the functional dependence of the viscosity on the shear rate.Before proceeding with the detailed argument, it is instructive to outline the mechanism of shear thinning in deeply supercooled liquids in qualitative terms, based on several key aspects of liquid dynamics in the activated regime established by the Random First-Order Transition (RFOT) theory (12): The high viscosity in supercooled melts arises because transient structures form, around each atom, that last longer than several hundred vibrations (13-15), thus rendering momentum transfer almost as efficient as in a fully stable solid. Consistent with this view, the v...

Viscous heating in silicate melts: An experimental and numerical comparison

“…Our findings challenge the traditional, and universal, view of the non‐Newtonian onset [ Vidal Russell and Israeloff , 2000; Berthier et al , 2005; Lubchenko , 2009] where this onset is attributed to cooperative structural rearrangement. Heterogeneous distribution of network modifiers (clustering) in silicate melts/liquids might promote stress localization and create structural non‐Newtonian behavior.…”

On the strength of glasses

“…The resulting additional enhancement of reconfiguration speed is a facilitation or mode coupling effect. Lubchenko has shown that this effect [that would be contained in a more complete RFOT theory including mode coupling effects (36)] does a good job describing the crossover to steady state nonlinear rheology (37). Similar effects have been studied in mode coupling treatments of dense colloid rheology (38,39).…”

Structural predictor for nonlinear sheared dynamics in simple glass-forming liquids

“…L iquids displaying slow dynamics, such as glass-forming liquids, exhibit striking nonlinear behavior known as shear thinning if subjected to sufficiently strong shear and the behavior is characterized by a power-law decrease of the viscosity η with increasing shear rateγ (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). Shear thinning is a process relied on heavily for, for example, industrial purposes such as lubrication.…”